1. Field of the Invention
The present invention relates to a structure of a feeder for a machine tool that is adapted to move a table relative to a saddle by means of a motor.
2. Description of the Related Art
A technology for a feeder having a structure in which a ball thread moves together with a table is disclosed, for example, in Japanese Patent Application Laid-Open No. 2001-300832. In a machine tool with such a feeder, a cover generally called a telescopic cover that expands and contracts during axial movement is used since a drive part needs to be protected against chips and cutting fluid and for the purpose of suppressing the size of the machine tool. A mechanism for expanding and contracting the telescopic cover may be a mechanism provided with stoppers at both ends of a movable area to suppress expansion and/or contraction, a mechanism that uses a pantograph for follow-up movement.
FIGS. 5 and 6 are views for explaining a structure of a feeder adapted to move a table relative to a saddle in a machine tool including the saddle and the table that moves relative to the saddle.
A ball thread nut 3b that engages with a ball thread shaft 2b is fixed on a lower surface (a surface facing a saddle 7b) of a table 6b. On the other hand, the ball thread shaft 2b is attached on an upper surface (a surface facing the table 6b) of the saddle 7b in a manner such that the thread shaft 2b engages with the nut 3b, is rotatable but is not movable in an axial direction of the thread shaft 2b relative to the saddle 7b. One end of the ball thread shaft 2b is coupled to a motor 1b fixed to the saddle 7b. 
Further, rails 4b along a linear axis are fixed on both the left and right portions of the upper surface of the saddle 7b, and bearings 5b along the linear axis that bear the rails 4b are fixed on both the left and right portions of the lower surface of the table.
In FIG. 6, a ball thread shaft 10b for moving the saddle 7b in a direction perpendicular to the axial direction of the thread shaft 2b relative to a machine base 14a (FIG. 5) engages with a ball thread nut 11b fixed to the saddle 7b and has one end coupled to a motor 9b. 
With the above-described structure, a motor drive part including the motor 1b and the ball thread shaft 2b moves together with the saddle 7b relative to the table 6b. Accordingly, it is necessary to attach to the table 6b a telescopic cover 8b, which expands and contracts with the movement of the motor drive part (the motor 1b and the ball thread shaft 2b) relative to the table 6b so as to always protect the motor drive part.
A common disadvantage of mechanisms for expanding and contracting a telescopic cover is that such a mechanism may be easily damaged and thus requires periodic maintenance since it needs to follow a rapid movement in an axial direction and reduce the impact thereof.
In addition, if a feeder for a machine tool having a first axis and a second axis as control axes, for example, is structured to move a saddle relative to a machine base in a first direction and to move a table relative to the saddle in a second direction perpendicular to the first direction, the first axis needs to be driven together with the structure of the second axis. Thus, the mass of a movable part on the first axis and that on the second axis typically differ widely from each other. Accordingly, due to the difference between the mass of the movable part on the first axis and that on the second axis, complex control for synchronizing the two axes is required and it is difficult to improve the precision of machining of a workpiece by the machine tool.